Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be placed in a wellbore to stabilize a formation and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could damage the formation. Casing joints are sections of steel pipe, which may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections are usually designed so that a seal is formed between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomer seal (e.g., an o-ring seal), a thread seal, a metal-to-metal seal formed proximate the connection, or similar seals known in the art.
One type of threaded connection commonly used to form a thread seal in oilfield tubulars is a wedge thread. In FIGS. 1A and 1B, a prior art connection having a wedge thread is shown. “Wedge threads” are characterized by threads, regardless of a particular thread form, that increase in width in opposite directions on a pin member 101 and a box member 102. The rate at which the threads change in width along the connection is defined by a variable commonly known as a “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the threads to vary width along the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436 issued to Mallis, and assigned to the assignee of the present invention. That patent is incorporated herein by reference in its entirety.
Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference. Continuing with FIGS. 1A and 1B, on the pin member 101, a pin thread crest 222 is narrow towards the distal end of the pin member 101 while a box thread crest 291 is wide. Moving along an axis 105 (from right to left), the pin thread crest 222 widens while the box thread crest 291 narrows. In FIG. 2, the threads are tapered, meaning that a pin thread 106 increases in diameter from beginning to end while a box thread 107 decreases in diameter in a complimentary manner. Having a thread taper can improve the ability to stab the pin member 101 into the box member 102 and distributes stress in the connection.
Generally, thread seals are difficult to achieve with non-wedge threads having broad crests and roots, however, the same thread forms may have thread seals when used for wedge threads. Wedge threads do not have any particular thread form. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239 issued to Klementich, and incorporated herein by reference. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706 issued to Church, and incorporated herein by reference. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e. not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880 issued to Watts. The above thread forms are examples of thread forms that may be used for embodiments of the invention. Generally, open thread forms such as buttress or stub are not suitable for wedge threads because they would impart a large radial force on the box member. A generally square thread form, such as that disclosed by Watts, or a trapped thread form does not impart an outward radial force on the box member. Those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal may be accomplished as a result of the contact pressure caused by interference over at least a portion of the connection between the pin load flank 226 and the box load flank 225 and between the pin stab flank 232 and the box stab flank 231, which occurs when the connection is made-up. Close proximity or interference between the roots 292 and 221 and crests 222 and 291 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Generally, higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. The particular connection shown in FIG. 1A also includes a metal-to-metal seal that is accomplished by contact pressure between corresponding seal surfaces 103 and 104, respectively located on the pin member 101 and box member 102.
Wedge threads typically do not have a positive stop torque shoulder on the connection. For wedge threads that do not have a positive stop torque shoulder, the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more during make-up for a given torque range to be applied than for connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. “Selected make-up” refers to threading the pin member and the box member together with a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For tapered wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks come closer to each other (i.e. clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection, however, this also increases stress on the connection during make-up, which could lead to failure during use.
Before make-up, pipe dope is typically applied to both the pin member and the box member of a threaded connection. Pipe dope provides lubrication to aid the make-up of the connection and prevents galling to allow for the connection to be broken-out at a later time. In oilfield applications, the pipe dope typically contains metallic particles, such as copper, to prevent galling between the threads of the pin member and the box member. The metallic particles also help achieve a thread seal between wedge threads, which make-up on both the load and stab flanks.
Because of the close-fitting manner in which wedge threads make-up, as compared to a shouldered non-wedge thread connection, less pipe dope is required. Typically, the pipe dope is only applied to the pin thread of a wedge thread connection. The application of the pipe dope is also typically achieved with a brush instead of a large swab, as is typical of other non-wedge thread connections. When a wedge thread connection is made-up, excess pipe dope can become trapped between the pin thread and the box thread, which can cause false torque readings (leading to improper make-up) or potentially damage the connection. Many of the problems associated with the pipe dope can be mitigated by applying less pipe dope than non-wedge thread connections and controlling the speed at which the connection is made-up to allow for the pipe dope to squeeze out.
Actually damaging a connection as a result of pipe dope is rare, but is still a concern for operators. One scenario in which damage to the connection can occur is when the pipe dope is too viscous. This can occur in cold weather environments such as North Slope Alaska or the North Sea when the wrong pipe dope is used. For cold environments, pipe dope with lower metal content and reduced kinematic viscosity is supposed to be used. Kinematic viscosity is the ratio of the viscosity of a fluid to its density. Centistoke is a common unit for kinematic viscosity. A centistoke is the viscosity in centipoise divided by the liquid density at the same temperature. If the wrong pipe dope is used and the connection is made-up quickly, as is typical of a power frame used for making-up connections, the pipe dope can become trapped between the pin thread and the box thread, causing a high pressure build-up that expands the box member.
A more common scenario that can occur when making up a wedge thread connection is pipe stand-off. Pipe stand-off refers to the situation in which a connection gives a false torque reading that indicates the connection is fully made-up based on a make-up torque, but is not fully made-up based on the relative position of the pin member and the box member. Often, pipe stand-off is difficult to detect on the rig at the time of make-up, and even a small amount can threaten the integrity of the connection. One cause for pipe stand-off in wedge thread connections is hydraulic lock resulting from inadequate evacuation of pipe dope. The pressure build-up may then bleed off during use, risking accidental back-off of the connection or hydraulic leaks. Pipe stand-off is a particular concern for larger diameter threaded connections, such as those greater than or equal to about 9⅝ inches diameter (24.4 cm). Dope evacuation is more difficult for larger diameter threaded connections because of the longer helical path for the pipe dope.
Furthermore, pipe stand-off may be particularly problematic in strings used at elevated downhole service temperatures (i.e., the temperature a tubular would expected to experience in service). Particularly, in high-temperature service (e.g., temperatures greater than 250° F., a steam-flood string, or a geothermal string), even a small amount of stand-off may be deleterious. For instance, if a made-up wedge connection with even a small amount of stand-off is deployed to a high-temperature well, the dope may flow out of the wedge thread connection and reduce the integrity of the thread seal.
A wide range of pipe dopes are commercially available. Pipe dope is typically a proprietary formulation of lubricant(s) and particulates. In general, higher particulate concentrations result in more viscous pipe dope, which helps to provide a thread seal in wedge thread connections. The base grease is also largely determinative of the final kinematic viscosity of the pipe dope. One company providing pipe dope for threaded connections is JET-LUBE®, Inc. (Houston, Tex., USA). One type of pipe dope provided by JET-LUBE®, Inc. is KOPR-KOTE®, which contains less than 10 percent by weight of copper as the particulate additive. KOPR-KOTE® is provided in an alternative formulation for arctic use, as are several other JET-LUBE® formulations. Higher temperature pipe dopes (“thermal grade”) from JET-LUBE® utilize a petroleum oil with a kinematic viscosity of 414 to 506 centistokes at 40 degrees C. The “arctic grade” pipe dopes utilize a calcium base grease with a kinematic viscosity of about 20 to 24 centistokes at 40 degrees C., which is much lower than the thermal grade. Another pipe dope is JET-LUBE® NCS-30, which is specifically marketed for use with wedge thread connections. That pipe dope does not contain metallic particulates. Instead, JET-LUBE® NCS-30 uses a proprietary formulation of chemically inert fibers as the particulate additive. Also, JET-LUBE® NCS-30 uses a calcium base grease similar to the arctic grade compounds to provide reduced kinematic viscosity.
Although many of the problems with making-up a wedge thread are avoided by using a pipe dope with lower kinematic viscosity and/or reduced metal content, a disadvantage to such a pipe dope is reduced sealing ability in the wedge thread. The operating environment in the wellbore is much hotter than the surface, which allows for the pipe dope to flow more easily and not aid in maintaining the thread seal in the wedge thread. In general, the higher the kinematic viscosity of the pipe dope, the better the resulting thread seal in the wedge thread.
In addition to pipe dope selection, mechanical solutions for relieving pressure build-up of wedge thread connections during make-up have been proposed. An example of a mechanical solution is disclosed in U.S. Pat. No. 6,050,610 issued to Enderle and assigned to the assignee of the present invention. The '610 Patent is incorporated herein by reference in its entirety. The '610 Patent discloses a wedge thread connection with a groove in a thread root. The groove provides an escape path for pipe dope during make-up of the wedge thread connection. Similar to the '610 Patent, U.S. Pat. No. 6,905,149 issued to DeLange discloses providing a groove in a thread crest to provide an escape path for pipe dope. However, the groove in the thread crest may prevent a wedge thread so equipped from sealing, as the groove provides a leak path for the pipe dope. Alternatively, in a two-step wedge thread, the thread crest groove may be provided on only one step such that the seal integrity of the connection is not compromised.
The pressure-relief grooves disclosed in the '610 and '149 Patents are limited in depth and width because larger grooves would reduce the strength of the threaded connection. Because of the limited size, pressure-relief grooves and other mechanical solutions to pressure build-up of wedge thread connections during make-up may fail to prevent connection damage and pipe stand-off for problematic connections, such as larger diameter wedge thread connections.